Tasks spawned by #[tokio::main] are not captured

[zaharidichev]

Since a usual pattern is to call console_subscriber::init() at the beginning of the main function, we end up initializing the subscriber after the task spawned by the tokio macro is created. We then end up in a situation where we cannot associate tracing events with any task. The result of that is printing a bunch of error messages to the console.

Related: tokio-rs/tokio#4206

[zaharidichev]

I guess one way to avoid that is to export a macro from the console that will function in a similar way to tokio::main but will init the subscriber before the task is spawned.

[hawkw]

ahhh, good catch...that makes sense! hmm. this is a problem that's kind of endemic with the #[tokio::main] macro; I've also seen people confused when an #[instrument] attribute on main doesn't do anything. it would be nice to have a solution for this, but just ignoring them is fine for now...

[hellow554]

Is there a workaround for that?

[hawkw]

Is there a workaround for that?

Rather than using #[tokio::main], use Tokio's Runtime::new() API, and ensure that console_subscriber::init() is called before creating the Tokio runtime.

For example, if you are currently writing something like this:

#[tokio::main]
async fn main() {
    console_subscriber::init();
    // actually do stuff
}

you could instead write

fn main() {
    console_subscriber::init();
    let rt = tokio::runtime::Runtime::new()
        .expect("tokio runtime should be created successfully");

    rt.block_on(async move {
        // actually do stuff
    });
}

This should get you the same runtime configuration as using #[tokio::main], but here, the console will be set up before the block_on method is called.

[hellow554]

What about something like #[tokio::main(tracing)] or

#[tracing]
#[tokio::main]
async fn main { }

?

[DragonDev1906]

How is this still open after 4 years? One of the most common patterns (that I've seen recommended at least and how it is done in all the examples) is to put #[tokio::main] before your main function and make it async. Yet the resulting task that executes the main future (which often does a bunch of things) doesn't even show up in tokio console.

#[tokio::main]
async fn main() {
    console_subscriber::init();
    println!("Hello, world!");
    loop {
        tokio::time::sleep(std::time::Duration::from_secs(1)).await;
        println!(".");
    }
}

Thus requiring the following fix/workaround, which isn't documented anywhere else as far as I could tell:

fn main() {
    console_subscriber::init();
    run();
}

#[tokio::main]
async fn run() { }